Remove note: setup.cfg *does* need [mypy] section (#10364)

As per #9940, a [mypy] section is now required in setup.cfg in order for config to be loaded from that file. However, the docs are out of date.

This PR removes that out of date clause.

Author: jamescooke

docs/source/config_file.rst

@@ -35,7 +35,7 @@ section names in square brackets and flag settings of the form
 `NAME = VALUE`. Comments start with ``#`` characters.
 
 - A section named ``[mypy]`` must be present.  This specifies
-  the global flags. The ``setup.cfg`` file is an exception to this.
+  the global flags.
 
 - Additional sections named ``[mypy-PATTERN1,PATTERN2,...]`` may be
   present, where ``PATTERN1``, ``PATTERN2``, etc., are comma-separated